Synthetic aluminosilicates are normally produced via sequential addition of an alkali metal aluminate and sodium silicate raw materials at a temperature below that which induces crystallization. This sequential addition precludes optimal stoichiometric conditions for pre-crystallization reactions especially when using a single, large crystallization vessel or reactor for the mixing of the raw materials.
Synthetic aluminosilicates such as zeolite-A having substantially uniform relatively small particle size is a an important industrial product with a variety of uses. Heretofore a process designed to provide such a product was attempted in which separate feed streams of aqueous sodium silicate and aqueous sodium aluminate were brought together at right angles to each other to form a stream which was passed vertically down through a pair of orifices to ensure good mixing, and through a carbon steel duct or conduit into the reactor in which the product was crystallized. While this process and feeding arrangement was operable, unfortunately the particle size uniformity was not as good as desired, and the mean particle size tended to be larger than desired. In addition, a considerable amount of caking and hangup of product took place in the feed system described.
After considerable experimentation a feed system and a set of operating parameters have been developed which are deemed to overcome the shortcomings of the previously tested feed system. This new feed system enables successful operation using large reactors, while at the same time achieving the production of synthetic zeolite product meeting a uniform mesh particle size specification without sacrifice of product quality in terms of calcium depletion rate, calcium exchange capacity, and water adsorption capability. Indeed, it is conceivable that the new technology of this invention will not only reduce mesh size significantly and uniformly, but in addition may allow for increased zeolite product quality.
In accordance with this invention the feed streams are mixed together using a cofeed system that provides a more homogenous small volume mixture that enters a larger crystallization vessel or reactor in the proper xe2x80x9creaction geometryxe2x80x9d and in an unimpeded fashion. These conditions minimize the formation of an incorrect mixture where unreacted raw materials can act as particulate binder leading to high mesh product. Consequently, a higher purity product is formed with high performance characteristics such as high water adsorption capability and/or high alkaline earth metal exchange capacity.
More particularly, synthetic zeolite is produced by force feeding separate aqueous streams of sodium silicate and of an alkali metal aluminate into coaxial alignment with each other. The coaxially aligned solutions are then introduced into in an open space in coaxial fashion with one of the streams being injected from a nozzle. The open space constitutes a mixing zone in which the two streams come together and are thoroughly mixed to produce a precrystallized mixture. This mixture then passes in unimpeded fashion through an entrance port and into a crystallization reactor or vessel. The system used for conducting this operation thus serves the dual function of being a feeding system and a mixing system whereby the silicate and aluminate streams interact with each other before entering the crystallization reactor or zone. Intimate mixing is assured by virtue of the relatively small volume of the mixing zone and the higher velocity of the stream emanating from the nozzle.
Among the advantages of this invention is that it enables the feeding of the two aqueous streams at elevated temperatures so that they come together and mix with each other while at an elevated temperature. This in turn enables shorter processing cycling times since the use of preheated incoming streams avoids the need for heating up a large volume of a precrystallization mixture within a large crystallization reactor or vessel.
In another of its embodiments this invention provides a process of producing a synthetic zeolite from a sodium silicate and an alkali metal aluminate. The process comprises cofeeding a stream of an aqueous sodium silicate and an aqueous stream of an alkali metal aluminate concentrically into an enclosed space constituting a mixing zone. Preferably the walls defining the mixing zone are vertically disposed or substantially vertically disposed, and the mixing zone is defined by a cylindrical wall such as the interior of a pipe or conduit. The mixing zone leads directly to a port or other opening in a reactor or vessel in which the zeolite is crystallized. This cofeeding is conducted such that (i) sodium silicate and alkali metal aluminate are mixed together and form a precrystallization mixture of uniform or substantially uniform composition in the mixing zone before passing into the reactor, and (ii) the resultant precrystallization mixture enters the reactor as an unimpeded flow. By xe2x80x9cunimpeded flowxe2x80x9d is meant that there is an open, unrestricted flow path (preferably a downward flow path) from the mixing zone which preferably is proximate to the cylindrical flange or other member defining the entrance into the reactor or vessel in which the crystallization occurs. Preferably, the entrance port is disposed at the top of the crystallization reactor or vessel, but in any event it is preferred that the diameter of the entrance port in such reactor or vessel be at least as large as the inner diameter of the mixing zone so that there is no shelf to impede the flow of precrystallization mixture into the reactor or vessel.
In the above process it is preferred that one of the above streams is injected vertically or substantially vertically downwardly into the mixing zone from a locus at or above the top of the mixing zone, and centrally disposed relative to the mixing zone. While either such stream can be the one that is injected in this manner, preferably the aqueous sodium silicate stream is the stream that is the so-injected stream.
Another embodiment of this invention is apparatus for feeding a solution of a sodium silicate and a solution of an alkali metal aluminate to a reactor or vessel in which the zeolite is crystallized. Such apparatus comprises:
a) a vertical or substantially vertical cylindrical conduit having a lower end and an outer diameter;
b) a nozzle having an upper end and a lower end and wherein the upper end of the nozzle is connected to the lower end of the conduit;
c) a control valve disposed in said conduit at a locus above the upper end of the nozzle;
d) a cylindrical duct having an upline section contiguous with a downline section, and a lower end portion, at least said downline section having an inner diameter greater than the outer diameter of the conduit, said downline section being coaxially aligned with said conduit and extending downwardly vertically or substantially vertically from the lower end of said nozzle to define a cylindrically enclosed open space terminating at the lower end portion of the duct, said lower end portion being adapted to connect with and form an unimpeded passage into an upper portion of said reactor or vessel, said upline section extending at a 0xc2x0 to 90xc2x0 angle relative to the vertical or substantially vertical cylindrical conduit, said upline section having a control valve disposed therein at a locus remote from said downline section.
It will be understood and appreciated that portions of the above-described apparatus are referred to individually. Even so, such portions in at least some cases can be in the form of a single unitary member or they can be composed of individual sub-units or elements which are connected together as by means of a threadable connection or the like. For example the conduct and the nozzle can be separate parts which are screwed, welded or otherwise bonded together to form a single entity, or they can be a unitary member to begin with. Similarly the contiguous upline section and downline sections can be a single unitary member i.e., a single straight duct or a duct bent at an angle of up to about 90xc2x0 from the axis of the original duct. Alternatively, the duct can be composed for example of a pair of straight ducts which are joined together by an elbow duct at an angle of up to about 90xc2x0 from the axis of one of the ducts.
The above and other embodiments and features of this invention will be still further apparent from the ensuing description, the appended claims, and the accompanying drawings.